Photomechanical polymers are a special class of smart polymers that are responsive to light and are capable of generating photo-directed motions and dimensional or shape alteration at the macro-scale level. The possibility of light-transduced mechanical work was first demonstrated in the literature in the mid-1960s, and since then, considerable effort has been undertaken in the synthesis of photoresponsive polymers and the characterization of their photomechanical output. The photo-directed motion and/or alteration of photomechanical polymers are driven by the collective molecular-volume change brought about by the structural rearrangement of chromophoric units upon appropriate irradiation. The chromophoric units in photoresponsive polymers are photochromic and have the unique ability to reversibly interconvert between two structural isomers (each with distinctly different optical and physical properties) under appropriate excitation conditions. Examples of chromophoric units can be found in photo-isomerizable molecules such as azobenzenes, spiropyrans, spirooxazines, diarylethylenes and fulgides. Azobenzenes are one of the most-utilized photochromic units because of their excellent thermal stability, resolved isomeric forms, and unique optical nonlinearities, as well as their ability to form surface reliefs when subjected to conventional or polarization holography. The resulting photomechanical output of a polymeric material is dependent not only on its optical properties (absorption wavelength, wavelength of exposure, polarization of exposure) but also on its molecular architecture and morphology (amorphous, crystalline, liquid crystalline) and thermomechanical properties, as well as the geometrical properties of the device, e.g. thickness of a cantilever.
Many photoresponsive polymers comprise liquid crystalline polymer networks (LCN; both glasses and elastomers), and recent reports have characterized the photomechanical and thermomechanical responses of LCN for comparatively large magnitude responses typified by bending of cantilevers or dramatic uniaxial contractions of thin films. Notably, a majority of these efforts have characterized the response of azobenzene-based LCN to exposure to UV light, which is known to decrease the order of the LCN through trans-cis photoisomerization and can result in an isothermal phase transition. UV-induced responses in azobenzene LCN are limited due to the need for multiple light sources to reverse the trans-cis isomerization.
Polyimides (PIs) represent an important class of heat-resistant polymers useful in a wide variety of applications due to their unique combination of physical properties, thermal stability, and processability, and photoresponsive polyimides and copolyimides have also been the subject of several recent reports. For example, PIs containing azobenzene in the backbone or side-chain have been investigated for photo-induced alignment in liquid crystal display (LCD), as well as nonlinear optical applications. Several cross-linked azopolyimides have also been described and reported to be photomechanically active.